The present invention relates to a method for producing self-polarized ferroelectric layers, in particular PZT layers, with a rhombohedral crystal structure. Furthermore, the invention relates to layers of this type.
Ferroelectric layers are employed in diverse ways in the semiconductor industry. By way of example, they are integrated as ceramic components in the form of PZT layers in a finely patterned manner in silicon ICs (integrated circuits). Other possible applications for ceramic components of this type may be actuators or the like. PZT ceramics is understood in a general form to mean ceramics comprising the lead zirconate/lead titanate system. PZT ceramics are distinguished by the fact that they can have either a rhombohedral or a tetragonal structure, depending on the composition.
In order that ceramics of this type can be used optimally, they must be in the ferroelectric phase, that is to say polarized. In the case of PZT layers with a tetragonal crystal structure, it is possible to achieve that polarization through self-polarization of the layers. PZT layers with the tetragonal crystal structure have, for example, a Zr content (zirconium content) of less than 50%, and preferably from 10 to 30%. A transition into the ferroelectric state takes place when the absolute temperatures of the PZT layers fall below the Curie temperatures for the material. This possibility is described for example in the paper xe2x80x9cSlater model applied to polarization graded ferroelectricsxe2x80x9d by Mantese, Schubring, Micheli, Catalan, Mohammed, Naik and Auner in the journal Appl. Phys. Lett., Vol. 71, No. 14, Oct. 6, 1997, pages 2047 to 2049.
PZT layers having the composition of the rhombohedral side of the phase diagram of the PZT ceramics which, for example, have a Zr content of greater than or equal to 50% have to be produced in an unpolarized fashion and subsequently be polarized by the application of an electric field in order to obtain the desired ferroelectric (pyroelectric) function. Since this requires a very high electric field, the polarization can only be effected in a costly manner. Furthermore, the high electric field brings about a high reject rate for the PZT layers, for instance due to the occurrence of electrical breakdowns or the like.
Thin Solid Films, Vol. 289 (1996), pages 29 to 33, discloses a method in which a self-polarized ferroelectric layer made of PZT ceramic is formed on a substrate made of SiTiO3. The ferroelectric layer comprises a Zr-abundant layer and a Zr-deficient layer, the Zr-abundant layer being applied as a buffer layer between the substrate and the Zr-deficient layer. The Zr-deficient layer has a tetragonal crystal structure. The Zr-abundant layer is not purely rhombohedral, but tetragonalized on account of latticed distortions.
Integrated Ferroelectrics, Vol. 20, pages 191 to 203, reveals a method in which a self-polarized ferroelectric layer made of PZT ceramic is formed indirectly via an intermediate layer made of lead lanthanate/lead titanate (PLT) on a substrate made of platinum. The self-polarized PZT layer is Zr-abundant and has a rhombohedral crystal structure.
Taking the cited prior art as a departure point, the present invention is based on the object of providing a simple method which makes it possible to produce self-polarized ferroelectric layers, in particular PZT layers, with a rhombohedral crystal structure. Furthermore, the intention is to provide a corresponding ferroelectric layer.
This object is achieved by means of a method for producing a self-polarized ferroelectric layer, which is characterized by the following steps:
a) provision of a substrate and heating of the substrate to a temperature T1;
b) application of a layer combination to the substrate, where the layer combination is formed from a metal-deficient layer with a cubic crystal structure and a Curie temperature TC1 and a metal-abundant layer with a cubic crystal structure and a Curie temperature TC2, wherein T1 greater than TC1 greater than TC2, and wherein a metal deficiency and a metal abundance of the layers refers to a presence of at least one metal from the fourth subgroup of the Periodic Table; and
c) ending of the heating of the substrate and cooling of the layer after the application process, so that the self-polarized layer is formed.
The method makes it possible in a simple manner to use a self-polarized layer for inducing the ferroelectric polarization in a material which otherwise does not become self-polarized. This means that it is possible to produce self-polarized ferroelectric layers, and in this case in particular PZT layers, with compositions of the rhombohedral side of the corresponding phase diagram.
The layer, in particular the PZT layer, is applied at the substrate temperatures which are higher than the Curie temperature of the layer. As a result, the layer grows with a cubic structure. Only after the actual application process, during the cooling of the layer, does the transition take place from the paraelectric cubic phase into the ferroelectric phase.
Self-polarized rhombohedral layers with a (111) texture have the advantage over self-polarized tetragonal layers with a (111) texture because the polarization is perpendicular to the substrate surface and the pyroelectric coefficient thereby becomes higher.
Any suitable material, for example platinum or the like, can be used for the substrate.
In a preferred embodiment of the method, the layer with a rhombohedral crystal structure can be applied in such a way that first a metal-deficient layer with a Curie temperature TC1 is applied to the substrate, and that afterward a metal-abundant layer with a Curie temperature TC2 is applied to the metal-deficient layer.
The relationship TC2 less than TC1 less than T1 holds true for the individual temperatures. In this case, the metal deficiency and the metal abundance of the layers refers to the presence of at least one metal from the fourth subgroup of the Periodic Table.
In a preferred embodiment of the invention, which, however, should not be understood to be exclusive, the metal-deficient layer is designed as a Zr-deficient layer and the metal-abundant layer is designed as a Zr-abundant layer.
In order to assist understanding of the invention, the formation of ferroelectric layers on a Zr basis is therefore described hereinafter. However, the ferroelectric layers can also be formed on the basis of other metals from the fourth subgroup of the Periodic Table.
The production of the rhombohedral layers, in particular PZT layers, with induced polarization is carried out in such a way that first the Zr-deficient layer is applied to the substrate. After this layer has been applied, the parameters on the respective application apparatus (for example a sputtering apparatus as described in more detail further below) are changed during the application process in such a way that a Zr-abundant layer then grows. Once the layer has reached the desired total thickness, the application process is ended. At the same time, the heating of the substrate is also discontinued, for instance by switching off a corresponding substrate heating arrangement. The substrate now cools and the Zr-deficient layer is the first to reach its Curie temperature, change into the ferroelectric phase and, in the process, become self-polarized. In the course of further cooling, the temperature of the layer approaches the Curie temperature of the Zr-abundant layer. In this case, the polarization already present in the Zr-deficient layer induces the polarization in the Zr-abundant layer and the entire layer is self-polarized after the cooling process. In this case, the lower Curie temperature of the Zr-abundant layer in comparison with the Zr-deficient layer allows a higher pyroelectric coefficient to be expected.
The same inducing effect can also be achieved if the Zr-deficient layer is applied on the Zr-abundant layer.
In accordance with another embodiment of the invention, it is provided that the layer with a rhombohedral crystal structure can be applied in such a way that first a metal-abundant layer with a Curie temperature TC2 is applied to the substrate, and that afterward a metal-deficient layer with a Curie temperature TC1 is applied to the metal-abundant layer. The relationship TC2 less than TC1 less than T1 holds true for the individual temperatures. In this case, the metal deficiency and the metal abundance of the layers refers to the presence of at least one metal of the fourth subgroup of the Periodic Table.
Zirconium (Zr) can once again advantageously be used as preferred metal.
Still other variants are also conceivable with regard to the possibilities for arranging the metal-deficient and the metal-abundant layers. Thus, by way of example, a plurality of metal-deficient and metal-abundant layers may be applied alternately on the substrate. It is also conceivable for a metal-abundant layer to be arranged in a sandwich-like manner between two metal-deficient layers, and for this arrangement to be arranged above one of the two metal-deficient layers on the substrate. The invention is not restricted to the two embodiments described in more detail above.
The metal-deficient layer may advantageously be applied with a thickness of about 100 nm on the substrate and/or the metal-abundant layer.
The layer with the rhombohedral crystal structure may be applied with a total thickness of about 1 xcexcm. Layers having such a thickness are used as integrated pyrodetector arrays, for example.
In particular, it is advantageous if the metal-deficient layer is designed to be significantly thinner than the metal-abundant layer.
In a further configuration, first an electrode is applied to the substrate. The layer with a rhombohedral crystal structure is then subsequently applied to this electrode. The electrode may be connected to the substrate and the layer via suitable insulation and adhesion layers.
The layer with a rhombohedral crystal structure is advantageously applied to the substrate by means of a sputtering method. Sputtering methods are known from the prior art. By way of example, a multi-target sputtering method may be used.
When a sputtering method is used, a Zr-deficient layer may be sputtered onto the substrate. After that layer has been applied, the power levels on the targets are changed in the course of the deposition process in such a way that the Zr-abundant layer grows on the first applied layer, as has already been generally described above. Essentially, this involves increasing the power on the Zr and lead targets (if PZT layers are deposited).
The method as described above can advantageously be used for the production of self-polarized, ferroelectric, rhombohedral lead zirconium titanate layers.
In accordance with a further aspect of the present invention, a ferroelectric layer, in particular a PZT layer, is provided, which is self-polarized and which is arranged on a substrate. The ferroelectric layer is characterized in that the ferroelectric layer is formed from at least one metal-deficient layer with a tetragonal crystal structure and a Curie temperature TC1 and at least one metal-abundant layer with a rhombohedral crystal structure and a Curie temperature TC2, where: TC2 less than TC1 and where a metal deficiency and a metal abundance of the layers refers to the presence of at least one metal fro the fourth subgroup of the Periodic Table.
As a result, the disadvantages mentioned with regard to the prior art described above can be avoided. For the advantages, actions, effects and mode of operation of the ferroelectric layer, reference is hereby made to the entire contents of the above explanations concerning the production method.
In a further configuration, the metal-deficient layer may be arranged on the substrate and the metal-abundant layer may be arranged on the metal-deficient layer. In accordance with another embodiment the metal-abundant layer may be arranged on the substrate and the metal-deficient layer may be arranged on the metal-abundant layer. The other arrangement variants described above are also possible.
The metal-abundant and metal-deficient layers maybe designed as Zr layers.
The metal-deficient layer may advantageously have a thickness of about 100 nm. The ferroelectric layer may preferably have a total thickness of about 1 xcexcm.
An electrode may be provided between the substrate and the ferroelectric layer with a rhombohedral crystal structure.
The invention will now be explained in more detail using an exemplary embodiment with reference to the accompanying drawing.